1. Field of the Invention
The present invention relates generally to a method and apparatus for indicating parameters in an electrical system and, more particularly, but not by way of limitation, to a method and an apparatus for indicating the leakage resistance to ground in an alternating current electrical system.
2. Brief Description of the Prior Art
In the past, various monitoring, controlling and indicating devices have been constructed to sense, detect, indicate or measure various parameters of an electrical system, such as the leakage resistance or leakage current to ground, earth faults, overloads, conductor continuity and the like, for example. Most electrical power distribution systems have included devices designed to disconnect the load from the power source in response to a predetermined overload or the like. However, in general, such prior art devices have a relatively slow response time and, in most instances, such devices have disconnected the electrical power source from the load only after the condition had resulted in relatively severe damage to defective equipment.
For example, with respect to electrical systems including an electric motor, the insulation surrounding the motor windings may become worn causing a leakage current between the motor windings and ground via the worn or defective insulation and, when the leakage current increases to a certain high leakage current value, the motor windings short to ground resulting in relatively severe damage to the motor windings. Such severe damage to the motor windings generally results in a relatively expensive repair. In most instances, if the leakage current could have been detected before reaching such a relatively high value and if the motor could have been disconnected from the electrical power source substantially immediately upon detection of the leakage current, the defect causing the leakage current could be repaired in a substantially less expensive manner as compared to the repair expense involved in replacing the damaged motor windings. The above-discussed rationale would be applicable to the various other components in the electrical system, such as the service conductors, the transformers, the electrical power source, the controller and the like, for example.
The leakage current in an alternating current electrical system causes an unbalanced condition between the individual service conductors and this unbalanced condition has been utilized in the past to indicate the leakage current in the electrical system. However, there is an inherent unbalanced condition in most alternating current electrical systems and this inherent unbalanced condition can affect the reliability of such systems. The U.S. Pat. No. 3,252,052, issued to Nash, disclosed a leakage current detection device utilizing the unbalanced condition in the service conductors to indicate leakage current. The U.S. Pat. No. 3,165,671, issued to Mintz et al., also disclosed a device capable of responding to a unbalanced condition in a polyphase system.
Some ground sensing devices constructed in the past have developed a direct current power source utilizing the available alternating current power source and the developed direct current power source has been utilized in the control ground sensing apparatus, such as the U.S. Pat. No. 3,058,035, issued to Brown. The U.S. Pat. No. 3,308,346, issued to Martzloff et al., disclosed a fault detector wherein a direct current was circulated through a conductive loop and a bridge circuit was utilized for comparing the value of the circulating direct current in the conductive loop to a reference value, a pilot conductor and a ground conductor being utilized to establish the conductive loop.
The U.S. Pat. No. 3,158,785, issued to Gagniere et al., disclosed a fault detector utilizing the unbalanced condition or a supervisory direct current injected between the network and ground. Other prior fault detectors, leakage detectors, ground detectors, safety circuits and the like were disclosed in the following U.S. Pat. Nos. 1,648,694, issued to Merrick; 1,826,965, issued to Stryker; 2,079,636, issued to Sharp; 2,147,101, issued to Leyburn; 2,157,886, issued to Cuttino; 2,224,320, issued to Schweitzer, Jr.; 2,238,570, issued to Schweitzer, Jr.; 2,428,563, issued to Fountain; 2,585,734, issued to Bucher; 2,660,717, issued to Hood; 2,697,217, issued to Jeffers; 2,700,125, issued to King et al.; 2,844,765, issued to Sosnoski; 2,883,610, issued to Brown; 3,187,225, issued to Mayer; 3,202,875, issued to Bateman; 3,213,321, issued to Dalziel; 3,214,638, issued to Moser; 3,229,163, issued to Rogers, Sr.; 3,335,324, issued to Buckeridge; 3,505,566, issued to Conrad; 3,515,941, issued to Moore et al.; 3,617,836, issued to Henry; 3,638,072, issued to Kobayashi et al.; 3,697,809, issued to Self; 3,754,221, issued to Stelter.